Leaching rests are a means to determine ifa radioactive waste package is acceptable for final disposal in accordance with the requirements of the regulation authorities. The relation between the leaching behavior of solidified waste with hydraulic binders or polymer matrices in the following different testing conditions has been examined. 1. French standard leaching tests (immersion in a specific volume of shallow land-disposal groundwater) 2. lysimeter experiments in various hydraulic environments with lower water contents. The latter conditions are more representative of repository conditions for waste forms in shallow land disposal than the standard test and therefore allow a better under, standing of the phenomena to be analyzed. Real lysimeter experiments are complex, and their relations with a standard test are even more so; therefore, after a br ief survey of the generally published works confirming this complexity, the choice of an approach starting from a step-by-step simulation is defined, and the experiments are presented in order of development. The results generated for cemented wastes, while incomplete, provide the following conclusions: 1. Activity fractions released are little different (maximum difference is 25%) between the two following types of tests. a. the type of test where waste forms are placed in contact with an inert simulated backfill material with different moisture contents (from a 35% volumetric water content in saturated state to similar to 2% in unsaturated equilibrium) and with sequential renewal of the leachant (by fast saturation for 15 min and draining for 30 min at the beginning of each sequence) b. the reference case derived from the French standard leaching test (by means of sequentially renewed periods of soaking only in a typical volume and representative composition of groundwater). 2. A rough preliminary trial of waste forms in a silica sand backfill led to the same conclusion as the results of waste forms release in a backfill material simulated by glass beads. 3. Moreover, with a high steady flow rate of water directly around the waste form sample and then 10 to 100 times the standard test water volume, the differences in leaching prove to be relatively slight and principally affected by the carbonate contents in the leachant synthetic groundwater, i.e., the carbonate content corning up from the CO2 of the environment during the test. The carbonation of the cemented waste can reduce the leaching so much that, in spire of the considerable water volume involved (10 to 100 times the standard amounts), the cesium leached fractions prove to be lower than the standard fractions. 4. Typical experiments with nearly dry waste environments are also in progress; they have already shown the following. a. Very small (leached factions of cesium migrate in this inert porous backfill with a low moisture content; in particular, on the glass beads 1 cm away (and beyond) from the embedded waste, the cesium fractions found are only <0.01 the leached fractions found in the water of the standard test. b. However, at the same time, almost as much cesium as during the standard leaching test becomes available (at the Surface of the cemented waste), being able to be quickly leached by simple rinsing or soaking, as if if is accumulated in quickly accessible parts of the cemented waste. Our current explanation of the very low cesium fraction in the nearly dry part of the backfill is that the diffusion area is very much reduced in this unsaturated material because the water distribution becomes very discontinuous in he large pores of such a backfill.
